It is a generally known principle of radio engineering that an amplitude-modulated signal can be demodulated by means of an envelope detector. An envelope detector is a relatively simple electronic circuit, the essential components of which are a diode, a capacitor, and a resistor. The circuit is laid out in such a way that the capacitor is allowed to be charged very quickly by the incoming signal over the forwardly polarized diode during at most a quarter-period of the incoming signal (e.g. the positive part of the rising edge of a carrier wave cycle), after which the capacitor discharges more slowly over the resistor for the remainder of the period, where the reversal of the diode's polarization inhibits a more rapid discharge. When the amplitude envelope decreases, the discharging phase continues until the voltage over the capacitor intercepts a rising edge of the incoming signal.
The electrical voltage over the capacitor thus roughly follows the peaks of the incoming signal, with a ripple determined by the time constant characteristic of the combination of the capacitor and the resistor. This time constant is given by T=RC, where R is the resistance of the resistor and C is the capacitance of the capacitor.
One more particular way of obtaining an envelope detector consists of combining a clamp circuit with an additional capacitor and a resistor, such that the incoming signal is first “clamped” to a certain non-zero bias level, which we can assume without loss of generality to be positive, after which the voltage over the capacitor, which has an averaging effect, approximately represents the envelope of the original incoming signal.
Amplitude-shift keying (ASK) is a digital modulation technique in which information is coded into an alphabet of symbols represented by bursts of a sinusoidal signal with different amplitudes. In its simplest form, binary ASK codes one bit per symbol, represented by two different amplitude levels. The ratio of the respective amplitude levels is a design choice. Optionally, one of these amplitude levels is zero; the resulting modulation scheme may be described as on/off signaling. Binary ASK may directly encode bit values ‘1’ and ‘0’ to different amplitude levels, or use a more complex coding scheme such as Manchester encoding, run-length encoding, and other alternatives known in the art.
Examples of wireless digital transmission systems that use binary ASK transmission are contactless smart cards and radio-frequency identification (RFID) systems. These are highly miniaturized systems. Some components of these systems are entirely passive, in that the component obtains both operating power and the signal to be received from the electromagnetic fields emitted by the transmitter. Others have their own operating power sources. These systems typically receive the ASK modulated signal from the transmitter via inductive coupling in the near field.
In a typical ASK reception circuit, the output of an envelope detector is passed through an analog-to-digital convertor (ADC) and fed into a digital circuit or component that processes the binary information contained in the received signal.